React:

- had a 3 year head start on Svetle (2013 vs. 2016)

- has major engineering orgs behind it. Facebook started it. Microsoft makes it work with Typescript.

- is easier to get started with. While JS-compilation is common, it's required for Svetle.

Svelte isn't free of pain points. Like any form of magic, it has edge cases. It's better but not "better enough" to de-throne React.